International Journal of Emerging Engineering Research and Technology Volume 2, Issue 3, June 2014, PP 201-212 ISSN 2349-4395 (Print) & ISSN 2349-4409 (Online)

Design and Construction of 1kVA Inverter 1,*

Babarinde, O. O., 1Adeleke, B. S., 1Adeyeye, A. H., 1

Ogundeji, O. A., and 2Ganiyu A. L. 1

Electrical Engineering Department The Polytechnic, Ibadan, Adeseun Ogundoyin Campus, Eruwa, 2 AET Company, Nigeria [email protected] Abstract: The design and construction of a 50HZ, 240V 1kVA inverter is primarily based on an inverter circuit which inverts the D.C. source voltage from a battery, AC voltage for AC powered appliances. The overall operation of this system comprises inter connections of many sub-circuits to give optimum performances. The sub circuit include; the oscillator circuit, PWM circuit, driver circuit, low battery/overload shutdown circuit, charging control/soft charging circuit, surge protection circuit, changeover/power supply circuit, and the output circuit (MOSFET and transformer section). This project incorporates monitoring circuit that employs visual display components such as light-emitting diodes and voltmeter to communicate the state of the system to the user.

Keywords: transformer, MOSFET, light-emitting diode (LED), battery, optimum performance

1. INTRODUCTION An inverter is a device that changes D.C. voltage into A.C. voltage. A direct current (D.C) is a current that flows in only one direction, while an alternating current (A.C.) is that which flows in both positive and negative directions. At the early stage, sun was the source of energy for generating power. Due to the inadequacy of the power generated through this source, there was a need to find other ways to improve the power supply when the generating station could not meet the demand of the people. As the technology advances, the hydroelectric generation was developed, gas firing generating station, and wired tubing methods of generating power supply were developed. In spite of all these developments, there was still failure in electrical power generation as a result of obsolete equipment at the generating stations. There was still need to find alternative for solving the problem. As a result of this, some options like alternators, inverters and others were developed. The electrical inverter is a high power electronic oscillator. It is so named because early mechanical AC to DC converters was made to work in reverse, and thus was “inverted”, to convert DC to AC. The inverter performs the opposite function of a rectifier formed in the late nineteenth century through the middle of the twentieth century; DC to AC power conversion was accomplished using rotary converters or motor-generator sets (M-G set). In the early twentieth century, vacuum tubes and gas filled tubes began to be used as switches in inverter circuits. The most widely used type of tube was the thyratron. The origins of electromechanical inverters explain the source of the term inverter. Early A.C to D.C converters used an induction or synchronous AC motor direct – connected to a generator (dynamo) so that the generators commutator reversed its connections at exactly the right moments to produce DC. A later development is the synchronous converter, in which the motor and generator windings are combined into one armature, with slip rings at one end and a commutator at the other end only one field frame. The result is either AC – in, DC – out with an M-E set, the DC can be considered to be separately generated from the AC, with a synchronous converter, in a certain sense it can be considered to be ©IJEERT

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mechanically rectified AC” Given the right auxiliary and control equipment, an M-G set or rotary converter can be run “backwards”, converting DC to AC. Hence an inverter is an inverted converter. There have been a large number of articles written concerning power conversion in recent years. This can be attributed in part to the rise in popularity of high voltage DC transmission systems. And their integration with existing AC supplies grids.

Transformer: It is an inductively coupled circuit used for transmitting alternating current energy. It is also used for matching impedance between the generator and the load. It makes use of mutual inductance in which a current flowing in a coil produces a varying electromagnetic wound over the primary coil. Most transformers are used to step-up or step down voltage or current. The number of turns on the primary winding is usually different form that of Secondary. However, an isolation transformer provides secondary voltage and current that is same as that of primary voltage and current, because both winding have the same number of turns, (Expect for resistive losses). These transformers prevent the transfer of unwanted electrical noise from the primary to the secondary winding. The primary and secondary windings of conventional transformer for electronic application are wound on tubular bobbin (insulated spool that serves as a support for the coil) made of plastic and other insulated materials. The wound bobbins are then enclosed by iron or steel cores in the shape of figure start of “E” and “I” shaped laminated metal sheets, assembled through and round the wound bobbins. The laminations are then clamped down to form a rigid assembly; some transformers have plastic shrouds to insulate the windings from the core. Both primary and secondary windings can be wound on the same bobbin, but it is now common practice, to wind the primary and secondary windings separately on a split bobbin, to improve electrical isolation. The primary and secondary terminals may be connected to rigid pins on the bobbin that also functions as printed circuit board mounting pins. Mosfet: Metal Oxide Semiconductor Field Effect Transistor (MOSFET) is a three terminal device they can be used either as an amplifier or as a switch. MOSFETs are classified as enhancement a depletion types. Battery: It is a d.c. power source to electrical or electronic equipment or devices that make use of it. Battery is being made available as direct source of energy. It is therefore necessary to select a reliable battery for optimum performance. Light Emitting Diode: It radiates optical energy when forward biased. LED is divided into base on the type of optical energy it radiates. The visible LED provides a useful means of indicating the state of a circuit and is therefore used as an indicator. In order to use visible LED as an indicating there is a need for the use of a protective resistor, which serves as a potential divider as shown below. The invisible/infrared LED radiates infrared light when forward biased. It is used in conjunction with the photodiode phototransistor to form a sensing system as in the remote control circuit.

2. INVERTER DESIGN AND CONSTRUCTION The inverter is a two operation modes device, the inverting and the charging modes. The inverting mode comprises of the oscillator, the driver, the output (MOSFET) section, the PWM section, low battery / overload protection circuit, and the transformer. The charging mode implements the transformer, the FET‟s (internal diodes) and the charging control circuit. A third operation mode is the changeover modes for switching between the two aforementioned modes at times of auto-back up for power failure and power restoration for the charging process. This mode implements a delay circuit, electromagnetic relays and power supply circuit.

3. TRANSFORMER DESIGN A transformer is a device that couples two AC circuits magnetically and provides electrical isolation between the circuits while allowing a transformation of voltage and current from one circuit to another i.e. it is mainly used for voltage and current transformation and hence we made use of current voltage transformers in this project. International Journal of Emerging Engineering Research and Technology

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The Generated E.M.F in a Wounded Transformer

In observing an ideal transformer with secondary opened and the primary connected to a sinusoidal alternating voltage V1, the potential different causes an alternating current to flow in the primary since primary coil is purely inductive and has no output but draw the magnetizing current I only .And this I will function as to magnetized the core in the secondary .It is small in and lag voltage V1 by 900. It therefore produces an alternating flux that is proportional to the current inputs. This flux is linked by both primary and secondary windings .Thus; this leakage(s) produced a mutually induced e.m.f E 2 in secondary winding that anti-phase with V1 and has magnitude proportional to rate of change of flux and the number of secondary turns. Let N1=Number of turns in primary, N2=number of turns in secondary The transformation ratio, K= N1/N2 The equation for the voltage and current transformation of a transformer is given by K= V2/V1= I1/I2 F=Frequency of A.C input (Hz) Maximum flux in core (Webbers) =B*A Average rate of change of flux =maximum. Flux divided by 1/4F.

(Wb/s or Volt.)

Now rate of change of flux per turn means induced e .m .f in Volt Thus Average e.m. f /turn =4*F*max. Flux Since the flux is sinusoidal,

r.m.s =form factor*Average e.m.f /turn

But form factor =r.m.s value /Average value=1.11, Then r.m.s value of E.m.f =1.11*4F*max. Flux =4.44F*max. Flux , But max .flux =Bm*A r.m.s value of E.m .f in primary turn (Tp) =4.44F*Bm*A*Tp NOTE: Bm is assumed to be 15000Wb/m. F=50 Hz By introducing stacking factor (10-8) and Tp factor (0.9) then we have Number of turns per volt, NT.V-1= 7/A 4. CHOICE OF TRANSFORMER’S COMPONENTS The power Rating for the Inverter transformer (KVA) =1.0KA , E 2=12V Assuming the efficiency of transformer =85% Then Input rating =output /Efficiency=1000VA/0.85=1176VA Ip = PI / VP

VP = 260V

Ip = 1176 / 260 = 4.5A Is = Po / Vs

Vs = 12V

Ip = 1000 / 12 = 83.3A For practical design of inverter transformer Number of turns per volt for both primary and secondary winding is given by; NT.V-1= 7/A Where A is the area of transformer former in sq. inch Former area A is 2.3inch by 1.5inch = 3.45sq.inch NT.V-1= 7/3.45=2.03 International Journal of Emerging Engineering Research and Technology

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NT.V-1 = 2 (approximate value). Primary Winding Charger tapping winding turns Np1= NT.V-1 * E1

E1=220V

Np1 = 2 * 220 = 440turns Inverter (out) tapping winding turns NS2= NT.V-1 * E3

E3=260V

NS2 = 2 * 260= 520turns Difference of Inverting and Charging turns = 520 – 440 = 80turns. For the primary windings, charging tapping is brought out after 440 turns and an addition 80 turns is made for the inverter out tapping. Secondary Winding Secondary turns Ns= NT.V-1 * E2 E2 = 12V Ns= 2 *12= 24turns. (Bifilar winding) SWG Estimation Standard Gauge Weight, SGW, can be estimated as follow; Considering conduction current density J (with fixed value of 2.5A/mm2) and windings coil current. For Ip = 4.5A, the corresponding guage from tables is 24SWG and For Is = 83.3A, the corresponding guage from tables is 13SWG Fig 9 shows the inverting transformer and its tapping voltage.

5. 50 Hz FREQUENCY OSCILLATOR SECTION The generation of 50 Hz frequency by the oscillator section is based on the application of a PWM controller IC SG3524. The IC has the following under listed features which makes it excellent for the application. Complete PWM power control circuitry Single ended or push – pull outputs Line and load regulation of 0.2% 1% maximum temperature variation Total supply current is less than 10mA Operation beyond 100KHz Application 0˚C to +70˚C

6. SG3524 DESCRIPTION It is a monolithic IC that contains all the control circuitry for a regulated power supply inverter or switching regulator. Included in a 16-pin dual – in – line package is the voltage reference, error amplifier, oscillator, pulse width modulator PWM, pulse steering flip-flop, dual alternating output switches, current limiting and shutdown circuitry. Fig. 1 below shows the pins definition of the IC.

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Design and Construction of 1kVA Inverter

Fig. 1. SG3524 pin configuration

7. THEORY OF OPERATION Voltage Reference An internal series regulator provides a nominal 5V output which is used both to generate a reference voltage and is the regulated source for all the internal timing and control circuitry. This reference regulator may be used as a 5V source for other circuitry. It provide up to 50mA of current itself and can easily be expanded to higher current with an external PNP Oscillator The oscillator in the SG3524 uses an external resistor (RT) to establish a constant charging current into an external capacitor (CT). While this uses more current than a series-connected RC, it provides a linear ramp voltage on the capacitor which is also used as a reference for the comparator. The charging current is equal to 3.6V ÷ RT and should be kept within the approximate range of 30µA to 2mA; i.e. 1.8k